[throwawayffffas]

I am not an accountant or anything but I think a rule of thumb is employment costs 50% more than the employees salary. Don't feel too bad asking significantly more, after all you are not getting any equity which sounds like your main motivation.

[mandelbrotwurst]

That sort of rule of thumb is going to be most accurate for average salaries, because large amounts of the non-salary costs are relatively fixed (e.g. health insurance costs the same amount whether you earn $50K or $200K annually). As salary goes up, so does its share of total compensation.